Programmable broadband gain amplifiers (PGA) are commonly implemented as indicated in FIG. 1 with an input amplifier gain stage (IS) 10 that is followed by a programmable stepped attenuator (ATT) 20 that is followed by an output amplifier gain stage (OS) 80.
Such a topology is quite useful. At low attenuation levels, this topology allows separate blocks to independently control the most important specifications of the system. If the system is properly designed, at low attenuation levels, the noise figure is set by the input stage IS; and linearity and output swing are set by the output stage OS. The gain is controlled by the attenuator ATT.
Problems arise at lower gain settings when the programmed attenuation level of the attenuator ATT is significantly higher than the gain of the input stage IS.
The first problem is related to the noise floor specification that many systems require to be maintained at a certain level at high attenuation. Assuming the attenuator ATT is implemented using a π, T or bridged T based resistive attenuator, then at high attenuation levels, the integrated noise power at the output of attenuator ATT is given by equation (1) and the integrated noise power at the output of the system is given by equation (2).NPATT=10*log(kTB)  (1)NPOUT=10*log(kTB)+10*log(GAINOS)+NFOS  (2)where GAINOS is the gain of the output stage OS and NFOS is a noise figure of the output stage OS. Since kTB is constant, the output noise power becomes mainly a function of the output stage OS parameters. This fact constrains output stage OS design flexibility.
The second problem arises from the fact that broadband gain amplifiers are commonly used to transmit time division multiple access (TDMA) signals. To conserve power, the broadband gain amplifier is required to be able to turn the amplifiers off in the “Transmit Disable” mode and turn them back on in the “Transmit Enable” mode. Turning amplifiers off and on causes transient disturbances at the output of the amplifier. The specification on Transmit Enable/Disable transient spurious emissions is especially stringent at high attenuation levels. To achieve linearity typically requires the output stage OS is to be biased with a high current with the result that this is a major contributor to output spurious emission.